Measuring devices of this type find particularly advantageous application in the field of microphotolithography (fabrication of microcircuits on a substrate of semiconductor material, for example typically silicon, by exposure, by means of light radiation, for example ultraviolet radiation, of a resist deposited on the surface of this substrate so as to detect, at each instant, the distance and/or inclination deviation (parallelism defect) of a portion of said semiconductor substrate relative to the image plane of the objective of the microphotolithography apparatus and to provide in real time the appropriate position corrections to the substrate.
Another very advantageous application of this type of measuring device relates to the three dimensional measurement of any surface having a shape close to a plane: the surface to be measured is moved precisely in a reference plane (horizontal plane) in two perpendicular directions so that the entire surface is scanned, the measuring device detecting the height and the local slope of the surface; by processing the measurements it is possible to reconstruct the surface in its entirety with greater precision than that which standard three dimensional measuring machines can provide.
The measuring devices employed in known microphotolithography apparatuses are based on several techniques, namely optical triangulation or profilometry, collimated beam analysis, interferometry, etc.
The technique of optical triangulation or profilometry seems to be particularly advantageous because of the simplicity of implementation that it allows, both as regards the optical means to be used and as regards the means for processing the signals resulting in the desired information.
The general principle of optical triangulation is simple: an optical projection system projects an image onto the surface to be measured, and the image, reflected or scattered by the surface, is picked up by an optical detection system; if the surface to be measured is moved or if its shape is modified, the image on the surface moves or is distorted; this movement or distortion is perceived by the optical detection system and the image on the focal plane is modified; the measurement, on the focal plane, of the deviation between the theoretical or initial image and the moved or modified image provides, by data processing, the information about the movement or the distortion of the surface to be measured.
Various optical measuring devices using optical triangulation designed to carry out said measurements are known, especially in microphotolithography apparatuses: see for example documents U.S. Pat. No. 4,356,392, U.S. Pat. No. 4,383,757, U.S. Pat. No. 4,650,983, U.S. Pat. No. 4,866,262, U.S. Pat. No. 5,191,200, U.S. Pat. No. 5,502,311 and U.S. Pat. No. 6,721,036.
However, these known devices have various drawbacks from the structural standpoint (for example moving optical parts, such as oscillating mirrors) and/or from the optical standpoint (for example microstructured image formation and detection) which render the measurements insufficiently precise (vibrations induced by the moving parts), while at the same time the means (in particular optical means) employed prove to be too complex and too expensive.